Generally, an automatic document feeder is usually integrated into the scanning apparatus for successively scanning a stack of documents. The automatic document feeder has a sheet input tray for placing the stack of documents. The automatic document feeder also has a sheet pick-up module for successively feeding the stack of documents from a sheet input tray to the internal portion of the automatic document feeder in a sheet-feeding direction. For allowing only one document to be fed into the internal portion of the automatic document feeder at each feeding time, the sheet pick-up module has a sheet separation roller and a separation pad. The separation pad is disposed under the sheet separation roller. The sheet separation roller may provide a frictional force to the document that is contacted with the separation pad. The frictional forces between the sheet pick-up module, the separation pad and the documents should be elaborately controlled. Generally, the frictional force between the sheet pick-up module and the document contacted with the sheet pick-up module is greater than the frictional force between the documents. In addition, the frictional force between the separation pad and the document contacted with the separation pad is also greater than the frictional force between the documents. As a consequence, only one document is allowed to be fed into the internal portion of the automatic document feeder at each feeding time. As the automatic document feeder is used for a long time, the separation pad is usually abraded, or even losses the function of separating documents.
For solving the above drawbacks, an automatic document feeder is disclosed. Such an automatic document feeder has a retard roller in replace of the separation pad. The function of the retard roller is similar to the separation pad. Generally, the retard roller is advantageous over the separation pad. When the document is transported across the region between the sheet pick-up module and the retard roller, the retard roller is rotated as the document is transported across the region. As such, different zones of the retard roller are abraded by the document for different feeding times. Due to the rotation of the retard roller, the abraded zones of the retard roller are distributed over the retard roller. Whereas, the separation pad has a specified abrade zone. As a consequence, the retard roller has a longer user life than the separation pad.
However, the retard roller and the separation pad have the similar drawbacks. For example, there is no mechanism for prompting the abraded condition of the retard roller or the separation pad. In a case that the retard roller or the separation pad is seriously abraded and has malfunction, the use of the retard roller or the separation pad may increase the possibility of causing the double feeding problem. Until the user finds that the retard roller or the separation pad is seriously abraded, several double feeding events have occurred, and some documents are not successfully scanned. The process of checking the double-fed documents is time-consuming. In addition, it is also the time-consuming and troublesome to re-scan the double-fed documents.